Lithographic apparatus and method

ABSTRACT

A substrate table to support a substrate on a substrate supporting area, the substrate table having a heat transfer fluid channel at least under the substrate supporting area, and a plurality of heaters and/or coolers to thermally control the heat transfer fluid in the channel at a location under the substrate supporting area.

This application claims priority and benefit under 35 U.S.C. §119(e) toU.S. Provisional Patent Application No. 61/365,268, entitled“Lithographic Apparatus and Method”, filed on Jul. 16, 2010, to U.S.Provisional Patent Application No. 61/384,663, entitled “LithographicApparatus and Method”, filed on Sep. 20, 2010, and to U.S. ProvisionalPatent Application No. 61/417,097, entitled “Lithographic Apparatus andMethod”, filed on Nov. 24, 2010. The contents of each of the foregoingapplications is incorporated herein in its entirety by reference.

FIELD

The present invention relates to a lithographic apparatus and a methodof compensating thermal variation.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. In an embodiment, the liquid isdistilled water, although another liquid can be used. An embodiment ofthe present invention will be described with reference to liquid.However, another fluid may be suitable, particularly a wetting fluid, anincompressible fluid and/or a fluid with higher refractive index thanair, desirably a higher refractive index than water. Fluids excludinggases are particularly desirable. The point of this is to enable imagingof smaller features since the exposure radiation will have a shorterwavelength in the liquid. (The effect of the liquid may also be regardedas increasing the effective numerical aperture (NA) of the system andalso increasing the depth of focus.) Other immersion liquids have beenproposed, including water with solid particles (e.g. quartz) suspendedtherein, or a liquid with a nano-particle suspension (e.g. particleswith a maximum dimension of up to 10 nm). The suspended particles may ormay not have a similar or the same refractive index as the liquid inwhich they are suspended. Other liquids which may be suitable include ahydrocarbon, such as an aromatic, a fluorohydrocarbon, and/or an aqueoussolution.

Submersing the substrate or substrate and substrate table in a bath ofliquid (see, for example U.S. Pat. No. 4,509,852) means that there is alarge body of liquid that must be accelerated during a scanningexposure. This requires additional or more powerful motors andturbulence in the liquid may lead to undesirable and unpredictableeffects.

In an immersion apparatus, immersion fluid is handled by a fluidhandling system, structure or apparatus. In an embodiment the fluidhandling system may supply immersion fluid and therefore be a fluidsupply system. In an embodiment the fluid handling system may at leastpartly confine immersion fluid and thereby be a fluid confinementsystem. In an embodiment the fluid handling system may provide a barrierto immersion fluid and thereby be a barrier member, such as a fluidconfinement structure. In an embodiment the fluid handling system maycreate or use a flow of gas, for example to help in controlling the flowand/or the position of the immersion fluid. The flow of gas may form aseal to confine the immersion fluid so the fluid handling structure maybe referred to as a seal member; such a seal member may be a fluidconfinement structure. In an embodiment, immersion liquid is used as theimmersion fluid. In that case the fluid handling system may be a liquidhandling system. In reference to the aforementioned description,reference in this paragraph to a feature defined with respect to fluidmay be understood to include a feature defined with respect to liquid.

One of the arrangements proposed is for a liquid supply system toprovide liquid on only a localized area of the substrate and in betweenthe final element of the projection system and the substrate using aliquid confinement system (the substrate generally has a larger surfacearea than the final element of the projection system). One way which hasbeen proposed to arrange for this is disclosed in PCT patent applicationpublication no. WO 99/49504. As illustrated in FIGS. 2 and 3, liquid issupplied by at least one inlet onto the substrate W, preferably alongthe direction of movement of the substrate W relative to the finalelement, and is removed by at least one outlet after having passed underthe projection system PS. That is, as the substrate W is scanned beneaththe element in a −X direction, liquid is supplied at the +X side of theelement and taken up at the −X side. FIG. 2 shows the arrangementschematically in which liquid is supplied via inlet and is taken up onthe other side of the element by outlet which is connected to a lowpressure source. In the illustration of FIG. 2 the liquid is suppliedalong the direction of movement of the substrate W relative to the finalelement, though this does not need to be the case. Various orientationsand numbers of in- and out-lets positioned around the final element arepossible, one example is illustrated in FIG. 3 in which four sets of aninlet with an outlet on either side are provided in a regular patternaround the final element. Note that arrows in FIGS. 2 and 3 show liquidflow.

A further immersion lithography solution with a localized liquid supplysystem is shown in FIG. 4. Liquid is supplied by two groove inlets oneither side of the projection system PS and is removed by a plurality ofdiscrete outlets arranged radially outwardly of the inlets IN. Theinlets and can be arranged in a plate with a hole in its center andthrough which the projection beam is projected. Liquid is supplied byone groove inlet on one side of the projection system PS and removed bya plurality of discrete outlets on the other side of the projectionsystem PS, causing a flow of a thin film of liquid between theprojection system PS and the substrate W. The choice of whichcombination of inlet and outlets to use can depend on the direction ofmovement of the substrate W (the other combination of inlet and outletsbeing inactive). Note that arrows in FIG. 4 show liquid flow.

In European patent application publication no. EP 1420300 and UnitedStates patent application publication no. US 2004-0136494, the idea of atwin or dual stage immersion lithography apparatus is disclosed. Such anapparatus is provided with two tables for supporting a substrate.Leveling measurements are carried out with a table at a first position,without immersion liquid, and exposure is carried out with a table at asecond position, where immersion liquid is present. Alternatively, theapparatus has only one table.

PCT patent application publication WO 2005/064405 discloses an all wetarrangement in which the immersion liquid is unconfined. In such asystem the whole top surface of the substrate is covered in liquid. Thismay be advantageous because then the whole top surface of the substrateis exposed to the substantially same conditions. This has an advantagefor temperature control and processing of the substrate. In WO2005/064405, a liquid supply system provides liquid to the gap betweenthe final element of the projection system and the substrate. Thatliquid is allowed to leak over the remainder of the substrate. A barrierat the edge of a substrate table prevents the liquid from escaping sothat it can be removed from the top surface of the substrate table in acontrolled way. Although such a system improves temperature control andprocessing of the substrate, evaporation of the immersion liquid maystill occur. One way of helping to alleviate that problem is describedin United States patent application publication no. US 2006/0119809. Amember is provided which covers the substrate in all positions and whichis arranged to have immersion liquid extending between it and the topsurface of the substrate and/or substrate table which holds thesubstrate.

SUMMARY

In an immersion lithographic apparatus, a liquid (e.g., water) ispresent between projection system and the substrate (e.g., wafer) toimprove imaging performance. The presence of liquid can lead to anadditional thermal load (e.g., a cooling load) compared to anon-immersion “dry” system. Such a thermal load may be applied directlyon the substrate (e.g., a thermal load, from a liquid supply system nearthe substrate, a thermal load due to evaporation of liquid on thesubstrate, etc) and/or may mainly affect the substrate table (e.g., thesubstrate table edge adjacent the substrate when the substrate is heldin a recess in the substrate table). In order to have good performance(mainly overlay and focus), it is desirable to have a substrate tablethat can reduce or eliminate the effects of such a thermal load in orderto avoid deformation (e.g., expansion or contraction) of the substrateand/or substrate table.

Thus, it is desirable, for example, to provide an apparatus in which theoccurrence of thermal expansion/contraction effects are reduced oreliminated. In particular it is desirable to provide a system configuredto reduce thermal expansion/contraction effects in an immersion systemwhich uses a liquid supply system which provides immersion fluid to thesubstrate and/or substrate table.

According to an aspect of the invention, there is provided a substratetable to support a substrate on a substrate supporting area, thesubstrate table comprising a heat transfer fluid channel under at leasta part of the substrate supporting area, and at least one heater and/orcooler at a location under the substrate supporting area to controlthermally the heat transfer fluid in the channel.

According to an aspect of the invention, there is provided an immersionlithographic projection apparatus comprising the substrate tabledescribed herein.

According to an aspect of the invention, there is provided a method ofcompensating for a local heat load in a lithographic projectionapparatus comprising a substrate table configured to support a substrateon a substrate supporting area, the method comprising controlling atleast one heater and/or cooler at a location under the substratesupporting area to thermally control a heat transfer fluid in a heattransfer fluid channel under at least the substrate supporting area.

According to an aspect of the invention, there is provided alithographic apparatus comprising a substrate table to hold a substrate;a reference frame; a grating; and a sensor, wherein the grating isattached to one of the substrate table and reference frame and thesensor is attached to the other of the substrate table and referenceframe, the sensor is configured to detect radiation diffracted and/orreflected by the grating so as to measure a relative position betweenthe substrate table and the reference frame, and the substrate tableand/or reference frame comprises a heat transfer fluid channel over orunder the grating and/or the sensor to control thermally the gratingand/or the sensor.

According to an aspect of the invention, there is provided alithographic apparatus comprising: a reference frame and/or a substratetable to hold a substrate; a grating attached to one of the substratetable and reference frame, wherein the grating is configured to diffractand/or reflect radiation detected to measure a relative position betweenthe substrate table and the reference frame; an optically transparentplate to cover at least a part of a surface of the grating; and a vacuumpump to maintain a gap between the grating and the plate at a pressurelower than an ambient pressure so as to hold the plate in position.

According to an aspect of the invention, there is provided alithographic apparatus comprising: a substrate table to hold a substrateon a substrate supporting area; and a grating or sensor attached to thesubstrate table, wherein the grating or sensor is a part of a positionalsystem, wherein the substrate table comprises a heat transfer fluidchannel under the substrate supporting area and adjacent the grating orsensor, so as to control thermally the temperature of the substratesupporting area and the grating or sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 schematically depicts a lithographic apparatus;

FIGS. 2 and 3 depict a liquid supply system for use in a lithographicprojection apparatus;

FIG. 4 depicts a further liquid supply system for use in a lithographicprojection apparatus;

FIG. 5 depicts, in cross-section, a fluid confinement structure whichmay be used in a liquid supply system;

FIG. 6 illustrates, in cross-section, a portion of a substrate tablesurrounding the edge of a substrate;

FIG. 7 illustrates, in plan, a substrate table;

FIG. 8 illustrates, in plan, a further substrate table;

FIG. 9 illustrates, in plan, a further substrate table;

FIG. 10 depicts a graph showing experimental performance of a substratetable;

FIG. 11 illustrates, in plan, a substrate table according to anembodiment of the invention;

FIG. 12 illustrates a temperature sensor according to an embodiment ofthe invention;

FIG. 13 illustrates a temperature sensor according to an embodiment ofthe invention;

FIG. 14 illustrates, in plan, the temperature sensor of FIG. 13;

FIG. 15 is a schematic drawing, in cross-section, illustrating anembodiment of the invention;

FIG. 16 is a schematic drawing, in cross-section, illustrating anembodiment of the invention;

FIG. 17 is a schematic drawing, in cross-section, illustrating anembodiment of the invention;

FIG. 18 depicts a variation of FIGS. 15 to 17;

FIG. 19 is a schematic drawing, in cross-section, illustrating anoptically transparent plate according to an embodiment of the invention;

FIG. 20 is a schematic drawing, in cross-section, illustrating anembodiment of the invention;

FIG. 21 is a schematic drawing, in cross-section, illustrating anembodiment of the invention;

FIG. 22 is a schematic drawing, in cross-section, illustrating anembodiment of the invention; and

FIG. 23 is a schematic drawing, in cross-section, illustrating anembodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

-   -   an illumination system (illuminator) IL configured to condition        a radiation beam B (e.g. UV radiation or DUV radiation);    -   a support structure (e.g. a mask table) MT constructed to        support a patterning device (e.g. a mask) MA and connected to a        first positioner PM configured to accurately position the        patterning device MA in accordance with certain parameters;    -   a substrate table (e.g. a wafer table) WT constructed to hold a        substrate (e.g. a resist-coated wafer) W and connected to a        second positioner PW configured to accurately position the        substrate W in accordance with certain parameters; and    -   a projection system (e.g. a refractive projection lens system)        PS configured to project a pattern imparted to the radiation        beam B by patterning device MA onto a target portion C (e.g.        comprising one or more dies) of the substrate W.

The illumination system IL may include various types of opticalcomponents, such as refractive, reflective, magnetic, electromagnetic,electrostatic or other types of optical components, or any combinationthereof, for directing, shaping, or controlling radiation.

The support structure MT holds the patterning device MA. The supportstructure MT holds the patterning device MA in a manner that depends onthe orientation of the patterning device MA, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device MA is held in a vacuum environment.The support structure MT can use mechanical, vacuum, electrostatic orother clamping techniques to hold the patterning device MA. The supportstructure MT may be a frame or a table, for example, which may be fixedor movable as required. The support structure MT may ensure that thepatterning device MA is at a desired position, for example with respectto the projection system PS. Any use of the terms “reticle” or “mask”herein may be considered synonymous with the more general term“patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device MA may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more patterning device tables). Insuch “multiple stage” machines the additional tables may be used inparallel, or preparatory steps may be carried out on one or more tableswhile one or more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD configured to adjust theangular intensity distribution of the radiation beam. Generally, atleast the outer and/or inner radial extent (commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in apupil plane of the illuminator IL can be adjusted. In addition, theilluminator IL may comprise various other components, such as anintegrator IN and a condenser CO. The illuminator IL may be used tocondition the radiation beam, to have a desired uniformity and intensitydistribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device MA. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PS,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam BSimilarly, the first positioner PM and another position sensor (which isnot explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WT may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device MA, the patterningdevice alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam B is projected onto a target portion C at one time (i.e.a single static exposure). The substrate table WT is then shifted in theX and/or Y direction so that a different target portion C can beexposed. In step mode, the maximum size of the exposure field limits thesize of the target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam Bis projected onto a target portion C (i.e. a single dynamic exposure).The velocity and direction of the substrate table WT relative to thesupport structure MT may be determined by the (de-)magnification andimage reversal characteristics of the projection system PS. In scanmode, the maximum size of the exposure field limits the width (in thenon-scanning direction) of the target portion C in a single dynamicexposure, whereas the length of the scanning motion determines theheight (in the scanning direction) of the target portion C.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam B is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Arrangements for providing liquid between a final element of theprojection system PS and the substrate can be classed into two generalcategories. These are the bath type arrangement in which the whole ofthe substrate W and optionally part of the substrate table WT issubmersed in a bath of liquid and the so called localized immersionsystem which uses a liquid supply system in which liquid is onlyprovided to a localized area of the substrate. In the latter category,the space filled by liquid is smaller in plan than the top surface ofthe substrate and the area filled with liquid remains substantiallystationary relative to the projection system PS while the substrate Wmoves underneath that area. A further arrangement is the all wetsolution in which the liquid is unconfined. In this arrangementsubstantially the whole top surface of the substrate and all or part ofthe substrate table is covered in immersion liquid. The depth of theliquid covering at least the substrate is small. The liquid may be afilm, such as a thin film, of liquid on the substrate. Any of the liquidsupply devices of FIGS. 2-5 may be used in such a system; however,sealing features are not present, are not activated, are not asefficient as normal or are otherwise ineffective to seal liquid to onlythe localized area. Four different types of localized liquid supplysystems are illustrated in FIGS. 2-5. The liquid supply systemsdisclosed in FIGS. 2-4 were described above.

Another arrangement which has been proposed is to provide the liquidsupply system with a liquid confinement member which extends along atleast a part of a boundary of the space between the final element of theprojection system and the substrate table. Such an arrangement isillustrated in FIG. 5. The liquid confinement member is substantiallystationary relative to the projection system in the XY plane thoughthere may be some relative movement in the Z direction (in the directionof the optical axis). A seal is formed between the liquid confinementand the surface of the substrate. In an embodiment, a seal is formedbetween the liquid confinement structure and the surface of thesubstrate and may be a contactless seal such as a gas seal. Such asystem is disclosed in United States patent application publication no.US 2004-0207824.

FIG. 5 schematically depicts a localized liquid supply system with afluid confinement structure 12, IH. The fluid confinement structure 12extends along at least a part of a boundary of the space 11 between thefinal element of the projection system PS and the substrate table WT orsubstrate W. (Please note that reference in the following text tosurface of the substrate W also refers in addition or in the alternativeto a surface of the substrate table W, unless expressly statedotherwise.) The fluid confinement structure 12 is substantiallystationary relative to the projection system PS in the XY plane thoughthere may be some relative movement in the Z direction (in the directionof the optical axis). In an embodiment, a seal is formed between thefluid confinement structure 12 and the surface of the substrate W andmay be a contactless seal such as a fluid seal, desirably a gas seal.

The fluid confinement structure 12 at least partly contains liquid inthe space 11 between a final element of the projection system PS and thesubstrate W. A contactless seal, such as a gas seal 16, to the substrateW may be formed around the image field of the projection system PS sothat liquid is confined within the space 11 between the substrate Wsurface and the final element of the projection system PS. The space 11is at least partly formed by the fluid confinement structure 12positioned below and surrounding the final element of the projectionsystem PS. Liquid is brought into the space 11 below the projectionsystem PS and within the fluid confinement structure 12 by liquid inlet13. The liquid may be removed by liquid outlet 13. The fluid confinementstructure 12 may extend a little above the final element of theprojection system PS. The liquid level rises above the final element sothat a buffer of liquid is provided. In an embodiment, the fluidconfinement structure 12 has an inner periphery that at the upper endclosely conforms to the shape of the projection system PS or the finalelement thereof and may, e.g., be round. At the bottom, the innerperiphery closely conforms to the shape of the image field, e.g.,rectangular, though this need not be the case.

In an embodiment, the liquid is contained in the space 11 by the gasseal 16 which, during use, is formed between the bottom of the fluidconfinement structure 12 and the surface of the substrate W. The gasseal 16 is formed by gas, e.g. air or synthetic air but, in anembodiment, N₂ or another inert gas. The gas in the gas seal 16 isprovided under pressure via inlet 15 to the gap between fluidconfinement structure 12 and substrate W. The gas is extracted viaoutlet 14. The overpressure on the gas inlet 15, vacuum level on theoutlet 14 and geometry of the gap are arranged so that there is ahigh-velocity gas flow inwardly that confines the liquid. The force ofthe gas on the liquid between the fluid confinement structure 12 and thesubstrate W contains the liquid in a space 11. The inlets/outlets may beannular grooves which surround the space 11. The annular grooves may becontinuous or discontinuous. The flow of gas is effective to contain theliquid in the space 11. Such a system is disclosed in United Statespatent application publication no. US 2004-0207824.

Other arrangements are possible and, as will be clear from thedescription below, an embodiment of the present invention may be usewith any type of liquid supply system.

In an immersion lithographic apparatus, a substrate is often positionedin a recess within the substrate table. In order to account forvariations in the width (e.g., diameter) of the substrate, the recess isusually made a little larger than the maximum likely size of thesubstrate. Therefore there exists a gap between the edge of thesubstrate and the substrate table. FIG. 6 is a schematic cross-sectionthrough a substrate table WT and a substrate W of such an arrangement. Agap 5 exists between an edge of the substrate W and an edge of thesubstrate table WT. The gap 5 is at an outer area or edge of a recess inwhich the substrate W is placed during imaging. The substrate W can besupported on a substrate supporting area of the substrate table WT. Inan immersion lithography machine using a localized area liquid supplysystem, when the edge of the substrate W is being imaged (or at othertimes such as when the substrate W first moves under the projectionsystem PS, as described above), a gap 5 between the edge of thesubstrate W and the edge of the substrate table WT will pass under, forexample, the space 11 filled with liquid by the liquid supply system 12.This can result in liquid from the space 11 entering the gap 5. In otherliquid supply systems, liquid can enter the gap 5 at any time.

In order to deal with the liquid entering that gap 5, at least one drain10, 17 may be provided at the edge of the substrate W to remove liquidwhich enters the gap 5. In the embodiment of FIG. 6, two drains 10, 17are illustrated though there may be only one drain or there could bemore than two drains. The drains 10, 17 are, for example, annular sothat the whole periphery of the substrate W is surrounded.

The primary function of the first drain 10 is to prevent bubbles of gasfrom entering the liquid 11 of the liquid supply system 12. Any suchbubbles can deleteriously affect the imaging of the substrate W. Thesecond drain 17 may be provided to prevent any liquid which finds itsway from the gap 5 to underneath the substrate W. Such liquid underneaththe substrate W may, for example, prevent efficient release of thesubstrate W from the substrate table WT after imaging. As isconventional, the substrate W is held by a pimple table 30 comprising aplurality of projections 32. An underpressure applied between thesubstrate W and the substrate table WT by the pimple table 30 ensuresthat the substrate W is held firmly in place. However, if liquid getsbetween the substrate W and the pimple table 30, this can lead todifficulties, particularly when unloading the substrate W. The provisionof the second drain 17 under the pimple table 30 reduces or eliminatesproblems which may occur due to liquid finding its way underneath thesubstrate W. The first drain 10 may comprise an inlet 110 (which may bea continuous groove or a plurality of individual through holes) whichputs a chamber into fluid communication with the gap 5.

When liquid is supplied to the space between the projection system PSand the substrate table WT, liquid may come into contact with a numberof structures of the lithographic apparatus including, for example, thesubstrate W and substrate table WT. Often, the liquid may evaporate. Theevaporation of liquid leads to localized cooling. Localized cooling canresult in mechanical contractions of the substrate W and/or substratetable WT, which in turn may lead to overlay errors. As an example, thedrawing of gas and/or liquid through the inlet 110 can lead to theevaporation of liquid which has entered the gap 5 and/or is on thesubstrate W. As another example, the movement of the substrate Wrelative to a liquid supply system 12 during exposures can lead toresidual liquid on the substrate W and/or substrate table WT, whichresidual liquid may evaporate.

One way in which this phenomenon of localized cooling may be dealt withis to provide a channel for a heat transfer fluid (e.g., heat transferliquid) in the substrate table WT. The temperature of the substratetable can be maintained substantially constant in this way.Additionally, as disclosed in United States Publication No. US2008-0137055, a further thermal control device (e.g., a heater and/orcooler) may be used to control the temperature of the substrate tableand/or substrate in the vicinity of the gap. Therefore the particularthermal load which is generated at or near the gap may be compensatedfor by the use of that further thermal control device.

FIG. 7 illustrates one such arrangement. FIG. 7 is a plan view of thesubstrate support area of a substrate table WT. The inlet 110 isindicated. A central channel 200 for heat transfer fluid is provided.The central channel 200 follows a path under the position of thesubstrate W. The path of the central channel 200 is such that an evenheating and/or cooling can be applied by passing a heat transfer fluidthrough the channel 200. The temperature of the heat transfer fluidentering the channel 200 is detected by a first temperature sensor 210.The temperature of heat transfer fluid exiting the channel 200 isdetected by a second temperature sensor 220. A third temperature sensor230 may be provided in the channel 200 to detect the temperature at alocal point. A controller can be provided with data from the temperaturesensors 210, 220, 230 and can control the temperature of the heattransfer fluid using a heater/cooler 240 which is used to control theheat transfer fluid prior to the heat transfer fluid entering thechannel 200.

In order to, for example, deal with the excessive cooling which can begenerated by the first drain 10, a thermal control element 250 (e.g., aheater) may be provided. In an embodiment, the thermal control element250 may be a single element which is adjacent the inlet 110 and extendsaround the periphery (e.g., circumference) of the inlet 110. In anembodiment, the thermal control element 250 may extend around theperiphery of the pimple table 30 at a location close to the edge of thesubstrate W when held on the substrate table WT. In an embodiment, thepimple table 30 closely conforms in size to, for example, a 200 or 300mm wafer. The thermal control element 250 may be positioned underneaththe chamber 140 or on either side of the chamber 140, as illustrated inFIG. 6. There may be other appropriate positions for the thermal controlelement 250.

A fourth temperature sensor 260 may be provided. The fourth temperaturesensor 260 is provided in the vicinity of the inlet 110. A controller261 can use the information obtained from the fourth temperature sensor260 to control the power applied to the thermal control element 250.

FIG. 8 illustrates a further substrate table WT. A plurality of thermalcontrol elements 310, 322, 324, 330, 342, 344, 350, 360 similar tothermal control element 250 are provided. At least two of the pluralityof thermal control elements 310, 322, 324, 330, 342, 344, 350, 360 arepositioned along different segments of an edge of the substratesupporting area. That is, the periphery of the inlet 110 and/or pimpletable 30 is segmented and each segment has at least one thermal controlelement 310, 322, 324, 330, 342, 344, 350, 360 associated with it. Thiscan be seen most clearly with reference to FIG. 8 which illustrates inplan the substrate support area just like in FIG. 7. In contrast to FIG.7, instead of providing a single thermal control element 250 around theentire periphery, the periphery has been split into six sections orsegments. Each section or segment is provided with at least one thermalcontrol element 310, 322, 324, 330, 342, 344, 350, 360. In FIG. 8 forillustration purposes each section is provided with a differentcombination of thermal control elements 310, 322, 324, 330, 342, 344,350, 360. However, it will be appreciated that any combination ofthermal control elements may be used and indeed all of the sections mayhave the same thermal control element configuration or only some of thesections may have the same thermal control element configuration.

In the first segment 91, only a single thermal control element 310 ispresent. In the second segment 92 three thermal control elements 322,324 are illustrated. In the third segment 93, two thermal controlelements 330 are provided.

In the fourth segment 94, a thermal control element 342 covers theentire length of the segment 94. In FIG. 8 the fifth segment 95 has asingle thermal control element 350 which does not extend along the wholelength of the segment 95. In the sixth segment 96, two thermal controlelements 360 are present which, contrary to what is the case in thethird segment 93, do extend along the entire length of the segment 96.

As can be seen, the thermal control elements, as a group, extendsubstantially around the edge of the substrate supporting area, eventhough there may be gaps between neighboring thermal control elements.At least two thermal control elements will be non-parallel and thishelps ensure that not just portions on opposite sides of the edge of thesubstrate supporting area have associated thermal control elements (asin, for example, the arrangement of parallel thermal control elements inU.S. Pat. No. 7,304,715).

Each of the thermal control elements are shaped closely to conform, inplan, to the portion of the edge of the substrate supporting area withwhich they are associated, that is, the thermal control elements arenot, in plan, straight, they are curved (where the substrate is curved).Each of the thermal control elements may be elongate in the vertical orhorizontal direction as well as elongate peripherally as illustrated inFIG. 8. Any combination of horizontal and vertical thermal controlelement(s) and/or number of thermal control element(s) and/or locationmay be used in each segment.

In an embodiment, the plurality of thermal control elements areindividually controllable in the sense that at least two of theplurality of thermal control elements are independently controllable ofone another. However, it may be the case that two thermal controlelements in the same segment are controlled in unison. For example, inthe third and sixth segments 93, 96 of FIG. 8, the thermal controlelements 330, 360 may be controlled in the same way.

In an embodiment, the control of the thermal control elements is donebased on results of temperature sensors positioned within the segment.As can be seen in FIG. 8, each segment may be provided with a singlesensor 410 as is illustrated in the first segment 91. Alternatively oradditionally, more than one temperature sensor may be provided persegment. One such example is illustrated in the second segment 92 wherethere are three temperature sensors 420 positioned along the length ofthe segment 92. The precise position of the temperature sensors canvary, particularly the height of the temperature sensor within thesubstrate table and its radial location relative to the components ofthe first drain 10 and/or the pimple table 30.

By providing the plurality of thermal control elements which areindividually controllable, it is possible to account for local thermalvariations. For example, if the inlet 110 passes under a localizedliquid supply system, cooling is only likely to occur along a length ofthe inlet 110 which passes under the area covered by liquid. With thesystem of FIG. 8 it is possible to thermally control only that area andso maintain a more constant temperature of the substrate table WT andthereby reduce overlay errors. By providing the thermal control elementsin groups of segments, it is possible to carefully control the localtemperature.

A controller 201 may be provided to control the thermal power applied bythe channel 200 and each of the thermal control elements. The controllermay base the amount of power supplied to the channel 200 and/or eachthermal control element 310, 322, 324, 330, 342, 344, 350, 360 onsignals received from one or more temperature sensors. Alternatively oradditionally, the thermal power applied by the channel 200 and/or thethermal control elements may be based on the position of the substratetable WT under the liquid handling system. Therefore, for example, whenthe position of the substrate table WT indicates that a thermal load maybe applied at a certain location, a thermal control element at thatlocation may be energized to compensate. The controller 201 may be inthe form of computer software. The controller 201 may control aplurality of thermal control elements of a single segment as a group orit may control those thermal control elements individually.

The controller 201 attempts to maintain the measured temperature at agiven set point. The faster the response the better the performancewhich can be expected. The faster the thermal time constants, thesmaller the net maximum temperature change which will occur on theapplication of a heat load. The controller 201 may control the channel200 and/or thermal control elements based on feedback from temperaturesensors in at the thermal control elements as well as optionally atleast one of sensors 220, 230, 260. Feed forward control is possiblebased on the position of the liquid handling system 12.

The temperature sensors discussed above may be on or in the material ofthe substrate table WT. For example, the temperature sensors may bepositioned at a joint between two such parts thereby to be embedded inthe substrate table WT though that is not necessarily the case.Embedding the temperature sensor may provide much better thermalresponse than positioning a sensor in a channel 200. The temperaturesensors could be point sensors in which case it is likely that more thanone temperature sensor per segment will be needed. For any particulararrangement, the thermal response may be better if the response iscontrolled by the signal from three temperature sensors in a segmentrather than just one temperature sensor. An average of the sensorsshould be taken. The sensors may be connected in parallel or in seriesbut this does not really affect their performance. Either way will givean average of the temperature measurement. Alternatively the temperaturesensors could be a ribbon sensor which by its nature averages thetemperature over an area. The sensor could, for example, be a NTC sensor(that is a negative temperature co-efficient sensor) which is surfacemounted.

The position of the channel 200 and thermal control elements 250, 310,322, 324, 330, 342, 344, 350, 360 is chosen according to the exactdesign of the substrate table. For example, it may be known that oneparticular part of the substrate and/or substrate table experiences ahigher heat load than other areas. In that case, for example, thechannel 200 and/or a thermal control element 250, 310, 322, 324, 330,342, 344, 350, 360 can be located close to that area.

FIG. 9 is a plan view of the substrate support area of a substrate tableWT similar to that depicted in FIGS. 7 and 8. A central channel 200 forheat transfer fluid is provided. The central channel 200 follows a pathunder the position of the substrate W. The temperature of the heattransfer fluid entering the channel 200 is detected by a firsttemperature sensor 210. The temperature of heat transfer fluid exitingthe channel 200 is detected by a second temperature sensor 220. A thirdtemperature sensor 230 may be provided in the channel 200 to detect thetemperature at a local point. A controller 211 can be provided with datafrom the temperature sensors 210, 220, 230 and can control thetemperature of the heat transfer fluid using a heater/cooler 240 whichis used to control the heat transfer fluid prior to the heat transferfluid entering the channel 200.

A plurality of thermal control elements 101 (e.g., heaters or coolers)are provided. Each of these thermal control elements is similar tothermal control element 310 in FIG. 8, i.e., each extends around aportion of the periphery. While six thermal control elements are shown,another number or arrangement may be provided.

Further, a plurality of temperature sensor systems 102 are provided.Each of these temperature sensor systems is similar to the threetemperature sensors 420 provided for segment 92 in FIG. 8. In anembodiment, each temperature sensor system has three temperature sensors102 although a different number may be provided. A controller 105 canuse the information obtained from the various temperature sensors 102 tocontrol the thermal load applied by the channel 200 and/or the thermalcontrol elements 101.

At least one first drain temperature sensor 103 may be provided underthe first drain 10. At least one second drain temperature sensor 104 maybe provided under the second drain 17.

In this arrangement, the thermal control elements 101 have a fastresponse time (e.g., in the order of 3-5 seconds). This is possible, forexample, because the temperatures sensors 102 for the thermal controlelements 101 are very close to those elements and/or because the thermalcontrol elements 101 can change their applied thermal load quickly.

On the contrary, the channel 200 has a quite slow response time (e.g.,in the order of 15-20 seconds). This is because the heater/cooler 240can be located quite far from the channel 200 portion under thesubstrate W. For example, the heater/cooler 240 may be located on aportion of the substrate stage separate and less thermally controlledthan the substrate table WT. So, after heat has been transferred betweenthe heater/cooler 240 and the heat transfer fluid, the heat transferfluid takes time to reach the channel 200 portion under the substrate Wand then further takes time to travel through the substrate table WT toreach all the temperature sensors 220, 230. While the temperature sensorposition could be changed, this may not be optimal since the sensors220, 230 should be “downstream” of the thermal load acting on thesubstrate W and/or substrate table WT in order for the temperaturesensor to “see” it but not too close to the gap between the substrate Wand the substrate table WT in order not to be disturbed by the greaterthermal load likely at that location.

The long response time discussed above can have a drawback. For example,a system with a long response time is more difficult to control. Theamplitude of temperature is larger with a slow system. Additionally oralternatively, slow control may be detrimental to high substrateexposure throughput. The time needed to expose a substrate is in thesame order of magnitude as the time constant of the heater/coolercontrol. This means that thermal disturbance during a substrate cyclemay have an effect on the following substrate. Additionally oralternatively, control of a quite large surface area (e.g., thesubstrate supporting area) with a single heater/cooler can be lead toquite large temperature gradients over the surface.

An illustration is provided of the large temperature gradient drawbackas well as the effect on following substrate drawback. Referring to FIG.10, a bare substrate was experimentally tested as a closing substratefor a liquid supply system 12 of an immersion lithographic apparatus.With a bare substrate, the residual liquid on the substrate from theliquid supply system is more than with a typical production substrateand so more evaporation was likely to occur. Referring to FIG. 10, thegradient over the substrate table increases when the loadincreases—compare the line 500 representing the temperature of the heattransfer fluid entering the substrate table WT (e.g., sensor 210) withthe line 510 representing the temperature inside the substrate table WT(e.g., sensor 230). This gradient is very large with the first test orthermally conditioning substrate (CTC substrate) but continues withsubsequent substrates W. The temperature inside the substrate table(e.g., sensor 230) is normally controlled at a target temperature, overdifferent substrates. Within a run of substrates, the first substratesuffers from what happened to the CTC substrate in FIG. 10—a noticeablegradient occurs. These effects would likely adversely affect overlay.

FIG. 11 depicts an embodiment of the invention. The substrate table WTcomprises the heat transfer fluid channel 200 under at least a part ofthe substrate supporting area. The heat transfer fluid channel 200 maybe in the pimple table 30 of the substrate table WT, or may be inanother section of the substrate table WT. At least one heater and/orcooler 530 is provided at a location under the substrate supportingarea.

The heater and/or cooler 530 is provided to control thermally the heattransfer fluid in the heat transfer fluid channel 200. The heater and/orcooler 530 is in addition to the heater/cooler 240 which is used tocontrol the heat transfer fluid prior to the heat transfer fluidentering the channel 200. The heater/cooler 240 is not under thesubstrate supporting area, whereas the heater and/or cooler 530 is underthe substrate supporting area. In an embodiment, the heater/cooler 240is not present.

The purpose of the heater and/or cooler 530 under the substratesupporting area is to control the temperature of the heat transfer fluidin the heat transfer fluid channel 200 accurately. The temperature ofthe heat transfer fluid under the substrate supporting area can bechanged more quickly compared to the substrate table WT depicted in FIG.9 that has a heat transfer fluid channel 200 without any heater orcooler at a location under the substrate supporting area.

Furthermore, by having at least the heater/cooler 240 to control thetemperature of the heat transfer fluid prior to the heat transfer fluidentering the channel 200 and the heater and/or cooler 530 to control thetemperature of the heat transfer fluid in the channel 200, thetemperature gradient of the heat transfer fluid in the channel 200 isreduced compared to a channel 200 that has only a heater/cooler 240 tocontrol the temperature of fluid prior to the fluid entering the channel200.

The heater and/or cooler 530 may be inside the channel 200. The heaterand/or cooler 530 may be disposed on an inner surface of the channel200. Additionally or alternatively the heater and/or cooler 530 may bedisposed on an outer surface of the channel 200, or may be embeddedwithin the substrate table WT distal from the channel 200.

In accordance with an embodiment of the invention, referring to FIG. 11,a plurality of heaters and/or coolers 530 may be provided to thermallycondition the heat transfer fluid in the channel 200 at variouslocations under the substrate supporting area, thereby controlling thetemperature of the substrate table WT.

Further, a plurality of temperature sensors 540 may be provided toimplement the thermal control. Each of the plurality of temperaturesensors 540 may be associated with each of the heaters and/or coolers530. For example, each heater and/or cooler 530 is controlled by atemperature sensor 540 downstream therefrom. While a particulararrangement of heaters/coolers 530 and sensors 540 is shown in FIG. 11,other designs with different numbers and/or locations of sensors andheaters/coolers may be used. The plurality of temperature sensors 540may include the third temperature sensor 230 described above in relationto the construction depicted in FIG. 9.

In an embodiment, the heater and/or cooler 530 is controlled accordingto readings from at least one of the temperature sensors 540. Forexample, it may be desirable to maintain the temperature of thesubstrate supporting area at a target value. If the controller 211 readsa measurement from temperature sensor 210 that the temperature of theheat transfer fluid in the channel 200 is less than the target value,then the controller 211 controls the heater and/or cooler 530 to raisethe temperature of the heat transfer fluid. If the controller 211 readsa measurement from the temperature sensor 540 that the temperature ofthe heat transfer fluid in the channel 200 is greater than the targetvalue, then the controller 211 controls the heater and/or cooler 530 toreduce the temperature of the heat transfer fluid. In this way, theheater and/or cooler 530 is paired to at least one of the temperaturesensors 540.

Desirably, within a pair comprising a heater and/or cooler 530 and atemperature sensor 540, the temperature sensor 540 is downstream of theheater and/or cooler 530. The purpose of this is that the heater and/orcooler 530 changes the temperature of the heat transfer fluid thatsubsequently flows to the temperature sensor 540. In this way, theheater and/or cooler 530 and the temperature sensor 540 form a feedbackloop.

In an embodiment, the substrate table WT is provided with a plurality ofsuch pairs, wherein each pair comprises a heater and/or cooler 530 and atemperature sensor 540. The pairs may be positioned sequentially alongthe channel 200 such that the heater and/or cooler 530 of a downstreampair is downstream of the temperature sensor 540 of an upstream pair.Each pair may comprise a controller 211. Alternatively, the controller211 may be common to (i.e. shared by) a plurality of pairs.

In an embodiment, the pairs overlap such that the temperature sensor 540of an upstream pair is downstream of the heater and/or cooler 530 of adownstream pair. Whether the pairs are arranged sequentially, oroverlapping, the channel distance between the temperature sensor of onepair and the heater and/or cooler 530 of a downstream pair is less thanthe channel distance between that temperature sensor 540 and the heaterand/or cooler 530 of the same pair. The purpose of this is to allowsufficient channel distance between the heater and/or cooler 530 and thetemperature sensor 540 of a pair so as to reduce the number of pairsused to control the temperature of the heat transfer fluid in thechannel 200 sufficiently accurately, while reducing or minimizing thechannel distance between sequential pairs (or even effectively anegative distance in the case of the overlapping configuration). Thepurpose of this is to keep to reduce or minimize regions of heattransfer fluid that are outside of the feedback loops of the pairs.

The temperature sensor 540 and the heater and/or cooler 530 of adjacentpairs are very close to each other. In an embodiment, the controller 211controls the heater and/or cooler 530 of a downstream pair according tomeasurements read from the temperature sensor 540 of an upstream pair(i.e. the heater and/or cooler 530 and the temperature sensor 540 thatare close to each other). The purpose of this is to allow thetemperature in one particular portion of the channel 200 (i.e. where thetemperature sensor 540 is positioned) to be controlled accurately andvery rapidly. This form of control can allow more rapid control of thetemperature at that position because of the decreased distance betweenthe temperature sensor 540 and the heater and/or cooler 530. This formof control may be particularly effective in the case of the overlappingpairs configuration.

The channel 200 may follow a tortuous path. The tortuous path maycomprise a series of hairpin bends. In an embodiment, the temperaturesensor 540 and the heater and/or cooler 530 are positioned immediatelydownstream of a hairpin bend.

In an embodiment, the heater and/or cooler 530 is not paired with atemperature sensor 540. A plurality of heater and/or coolers 530 may becontrolled in response to temperature measurements read from a singletemperature sensor. This has an advantage that fewer temperature sensorsare provided in the channel 200. In an embodiment, there may be a singletemperature sensor 230 in the channel 200, or there may be notemperature sensor in the channel 200 under the substrate supportingarea at all. In this case, the heaters and/or coolers 530 are controlledin response to readings taken from the temperatures sensors 210 and 220.In this case, the purpose of the heaters and/or coolers 530 is to smooththe temperature gradient of the heat transfer fluid in the heat transferfluid channel 200.

In an embodiment, the heaters and/or coolers 530 are embedded in orattached to the substrate table WT. For example, the substrate table WTmay have recesses and/or holes into which the heaters and/or coolers 530are placed. Similarly, in an embodiment, the temperature sensors 540 areembedded in the substrate table WT. In an embodiment, theheaters/coolers and/or sensors are located in the substrate table WT bycreating a hollow in the undersurface of the substrate table by, forexample, drilling. The heaters/coolers 530 and/or sensors 540 can besealed in situ with glue.

In an embodiment, the heaters and/or coolers 530 may thermally controljust the heat transfer fluid and not the substrate table WT itselfdirectly. For example, the heaters and/or coolers 530 may be in thefluid stream. In an embodiment, the heaters and/or coolers 530 directlycontact and surround the fluid stream. For example, the heaters and/orcoolers 530 may be a cylindrical heater and/or cooler. This increasesthe surface area of the heater and/or cooler 530 in contact with theheat transfer fluid, improving the effectiveness of thermal transfer.

In an embodiment, the temperature at sensor 210 is controlled to asubstantially fixed setpoint temperature to compensate for temperaturedrift occurring of the incoming heat transfer fluid that is supplied tothe substrate table WT. In the substrate supporting area, a thermal loadwould be controlled by the heaters and/or coolers 530, that load may bemostly or entirely due to, for example, immersion liquid and/or a liquidsupply system 12.

Thus, by having the arrangement of heaters/coolers 530 and optionallythe sensors 540 described above, the response time to the detection ofthermal variation may be reduced. For example, by having heaters/coolers530 closer to the thermal loads, the response can be quicker and theadverse effect of thermal variation on following substrates W may beavoided/greatly reduced. Further, the closer the temperature sensor 540to the heater/cooler 530, the response time of the heater/cooler 530will be faster, so the heater/cooler 530 will be better controllable.Having a plurality of heaters/coolers 530 may also reduce thetemperature gradient (i.e., cause more uniform temperature over asurface) and have thermal control applied to where it is needed.

In an embodiment, instead of the multiple heaters and/or coolers 530, atwo phase thermal control system may be provided at least under thesubstrate supporting area of the substrate table WT. The two phasethermal control system may use a refrigerant designed to change phase atthe desired substrate table temperature (e.g., 22° C.). Such a systemwould have quick response time (assuming closely located temperaturesensors) and be effective for local temperature variations of a surface.

In a further embodiment, a top side core heater (e.g., a thin filmheater) could replace the channel 200 or be placed over the channel 200.In an embodiment, the top side heater comprises one or more metal filmsapplied over at least the substrate supporting area and which can becontrolled in a similar manner as the heaters and/or coolers 530. In anembodiment, a plurality of films may be provided, each controlling thetemperature of a different region of the substrate supporting area andeach being individually controllable. Such a system would have quickresponse time (assuming closely located temperature sensors) and beeffective for local temperature variations of a surface.

In a further embodiment, the heater/cooler 240 may be the onlyheater/cooler to thermally control the heat transfer fluid in thechannel 200, with no further heaters and/or coolers. The heat transferfluid flow may be increased to a level such that the response time isreduced to the order of 3-5 seconds. However, physical limitations mayprevent or limit the effectiveness of this solution. In an embodiment,the heater/cooler 240 is as close as possible to the substrate table WT.

FIG. 12 depicts a part of an embodiment of the invention. FIG. 12depicts how the temperature sensor 540 may be attached to the channel200 or to the substrate table WT. The part to which the temperaturesensor 540 is attached is given the reference numeral 131. Thetemperature sensor 540 is configured to measure the temperature of thepart 131. The part 131 may be, for example, the channel 200 or thesubstrate table WT.

The temperature sensor 540 may comprise a thermistor, or otherthermometer equipment. According to the construction depicted in FIG.12, the temperature sensor 540 is pressed directly against the part 131.A thermally conductive paste 132 may be provided intermediate thetemperature sensor 540 and the part 131. The paste may be a heatconductive glue. The temperature sensor 540 is connected to anelectrical assembly 134 via at least one wire 133. The electricalassembly 134 takes temperature readings from the temperature sensor 540.The electrical assembly 134 may be a PCB. In an embodiment, thetemperature sensor 540 is mounted directly onto the electrical assembly134 without the need of the wire 133.

A drawback of the construction depicted in FIG. 12 is that it can bedifficult to position the temperature sensor 540 at the precise locationwhere it is desired to measure the temperature. This is partly due tothe presence of the electrical assembly 134 on which the temperaturesensor 540 is mounted, or the presence of the wire 133 connecting thetemperature sensor 540 to the electrical assembly 134. A furtherdrawback is that the wire 133 puts pressure on the temperature sensor540. This can undesirably affect the temperature measurement taken bythe temperature sensor.

The temperature sensor 540 may be made of a semiconductor material. Thetemperature sensor 540 is configured to measure the temperature at asingle location.

FIGS. 13 and 14 depict an alternative to the construction of FIG. 12 toattach the temperature sensor 540 to a part 131. FIG. 13 depicts a sideview of the construction. FIG. 14 depicts a plan view of theconstruction.

The temperature sensor 540, which may be a thermistor, is attached tothe part 131 at the location at which the temperature is to be measured.At this location, the part 131 is coated with an electrically conductivecoating 141. Desirably, the electrically conductive coating is thermallyconductive. As is most clearly seen in FIG. 14, the electricallyconductive coating 141 takes the form of a pattern. The purpose of thepattern of the coating 141 is to allow the electrically conductivecoating 141 to be connected to the electrical assembly 134 at anappropriate position. For example, an appropriate position may be wherethere is more space for the electrical assembly 134 or for the wire 133to connect to the electrical assembly 134. For this purpose, the coating141 may comprise at least one elongate portion.

The electrically conductive coating 141 also provides electricalshielding to the part 131 and/or to the temperature sensor 540. In thisway, electrical shielding can be provided without any additionalproduction steps. Measurement signals from the temperature sensor 540can be read out via the electrical assembly 134, which may be connecteddirectly to the coating 141, or indirectly via a wire 133.

The temperature sensor 540 may be attached directly to the coating 141.The temperature sensor 540 may be embedded within the coating 141. In anembodiment, the temperature sensor 540 is connected to a coating 141 viaa thermally conductive adhesive (i.e. glue) 132. Desirably, the layer ofglue 132 is less than 10 μm thick.

A gap 142 may be provided between the temperature sensor 540 and thecoating 141. The purpose of the gap 142 is to prevent short-circuiting.The coating 141 is formed of two coating sections. Each section acts asan electrode to provide power to the temperature sensor 500 and/orreceive signals from the temperature sensor 500. The gap 142 separatesthe two coating sections from each other. The gap 142 may be filled withan electrically insulating material.

The thickness of the coating is less than 10 μm, less than 5 μm, lessthan 3 μm, or less than 1 μm.

The electrically conductive coating 141 may be made of platinum, or apredominately platinum alloy, for example. The coating 141 may compriseat least one of copper, aluminum, silver and gold.

FIG. 15 is a schematic drawing, in cross-section of an embodiment of theinvention. In the above description, it has been described how a heattransfer fluid channel 200 may be employed to control the temperature ofthe substrate table WT, particularly under the substrate supportingarea. However, the use of such a heat transfer fluid channel is notlimited to the context of controlling the temperature of a substratesupporting area of a substrate table WT. Another use of such a heattransfer fluid channel 200 is depicted in FIG. 15. The heat transferfluid channel 200 depicted in FIG. 15 is based on a similar concept tothe heat transfer fluid channel 200 depicted in FIG. 11 and describedabove. The heat transfer fluid channel 200 depicted in FIG. 15 tocontrol the temperature of a grating 50 or a sensor 20 on the substratetable WT may be common to the heat transfer fluid channel 200 depictedin FIG. 11 to control the temperature of the substrate supporting area.The channel 200 may be in the same chuck or support table. The channel200 may comprise the same liquid flow system. The channel 200 may forminterconnected flow systems.

FIG. 15 depicts a lithographic apparatus comprising a substrate tableWT, a reference frame RF, a grating 50 and a sensor 20. The grating 50is attached to one of the substrate table WT and reference frame RF. Thesensor 20 is attached to the other of the substrate table WT and thereference frame RF. FIG. 15 depicts the case in which the grating 50 isattached to the substrate table WT and the sensor 20 is attached to thereference frame RF.

The sensor 20 is to detect radiation diffracted and/or reflected by thegrating 50, so as to measure a relative position between the substratetable WT and the reference frame RF. This is a type of positionalmeasurement device used in a lithographic apparatus in which the grating50 and sensor 20 are mounted on different objects which are moveablerelative to one another and whose relative position is desired to bemeasured.

As depicted in FIG. 15, the substrate table WT and/or the referenceframe RF comprises a heat transfer fluid channel 200 over or under thegrating 50 or sensor 20 to control thermally a surface of the grating 50or the sensor 20. FIG. 15 depicts the case in which a heat transferfluid channel 200 is positioned over the grating 50 attached to thesubstrate table WT. FIG. 15 also depicts an embodiment in which a heattransfer fluid channel 200 is positioned under the sensor 20 attached tothe reference frame RF. One or more heaters and/or temperature sensorsmay be positioned in the channel 200 adjacent the grating 50 or sensor20 so as to control thermally the fluid inside the channel 200. In anembodiment, one or more heaters and/or temperature sensors are notadjacent the grating 50 or sensor 20. The channel 200 may be similar tothe channel depicted in FIG. 9 and described above.

A fluid of relatively high heat capacity, such as water, flows throughthe channel 200 adjacent the grating 50 and/or sensor 20. Thisstabilizes the temperature of the grating 50 or sensor 20. Stabilizationof the temperature of the grating 50 or sensor 20 helps to reducepositional error that would otherwise lead to overlay error. Thepositional error is caused by thermal deformation of a surface of thegrating 50 or sensor 20. Such thermal deformation is caused by a thermalload on the surface. A thermal load is applied to the surface if aliquid not the same temperature as the surface comes into contact withthe surface. For example, the liquid may evaporate, or in any casethermally equilibrate with the surface.

This is a problem for the grating 50 as depicted in FIG. 15 because thegrating 50 is located on the upper surface of the substrate table WTover which the fluid confinement structure 12 is located. The fluidconfinement structure 12 may become located over all or part of thegrating 50 during normal operation of the lithographic apparatus. Inthis case, immersion liquid can escape from the fluid confinementstructure 12 and be left behind on the grating 50 as a film or as adroplet. Of course, the same problem can occur if the sensor 20 ispositioned on the top surface of the substrate table WT and the grating50 is positioned on the reference frame RF. The grating 50 may comprisea grid plate and a grating surface formed on an under side of the gridplate. The purpose of this is to prevent the grating surface itself fromcoming into contact with the immersion liquid. A thermal load on thegrating 50 or sensor 20 can be caused by reasons other than immersionliquid on its surface. For example, a thermal load may be caused by warmgas on or adjacent the grating 50 or sensor 20.

The lithographic apparatus may further comprise an optically transparentplate 262 configured to cover at least a part of a surface of thegrating 50 or the sensor 20. As depicted in FIG. 15, the plate 262covers substantially a whole upper surface of the grating 50. The plate262 may be made of any optically transparent material. In particular,the plate 262 may be made of a glass, or a glass-ceramic, for example.

As depicted in FIG. 15, the heat transfer fluid channel 200 may bepositioned between the plate 262 and the grating 50 (or the sensor 20).As such, the fluid (e.g. water) in the channel 200 runs in the thin gapbetween the grating 50 and the plate 262.

The plate 262 may cover the grating 50, which is covered by heattransfer fluid, and the surface of the substrate table WT surroundingthe substrate supporting area. The temperature of the heat transferfluid can be conditioned thermally to match the temperature of theimmersion liquid confined by the fluid confinement structure 12. When adroplet lands on the plate 262 above the grating 50, the heat load ofthe evaporating droplet is absorbed by the heat transfer fluid betweenthe plate 262 and the grating 50. Therefore, the flow of heat transferfluid under the plate 262 at least reduces, or possibly removes, theheat load that would otherwise be applied to the grating 50. The flow ofheat transfer fluid thermally conditions the grating 50.

Although the above description has focused on the grating 50, the sameadvantages and mechanisms are applicable to temperature control of thesensor 20. Furthermore, the channel 200 may be positioned below thegrating 50 or the sensor 20. The channel 200 may be positioned above thegrating 50 or the sensor 20. The arrangement of one or more heatersand/or temperature sensors in the channel 200 may be as described abovefor the channel 200 depicted in FIG. 11.

The plate 262 may have a lyophobic (e.g., hydrophobic) surface. Thelyophobic surface may take the form of an optically transparent coating.The coating may be as described in United States patent applicationpublication no. US 2009/206304. The plate 262 may be replaced easily.

The heat transfer fluid channel 200 may be connected to a separatethermal conditioning system beneath the substrate table WT. One or morethermal sensors and/or heaters, such as one or more thin film sensorsand/or heaters, may be integrated into the transfer fluid channel 200.For example, such a sensor and/or heater may be integrated into asurface of the channel 200 to facilitate the thermal conditioning. Thechannel 200 may be integrated with a channel 200 as depicted in FIG. 9or 11 to control the temperature of the substrate supporting area of thesubstrate table WT. The channel 200 for the grating and/or sensor andthe channel 200 for the substrate supporting area may be connected tothe same thermal conditioning system. The heater and/or temperaturesensor in the channel may be one or more thin film heaters and/ortemperature sensors as described with respect to FIG. 11 or in U.S.patent application no. U.S. 61/416,142.

FIG. 16 is a schematic drawing, in cross-section, of a lithographicapparatus according to an embodiment of the invention. As depicted inFIG. 16, the substrate table WT is configured to support a substrate Won a substrate supporting area. The optically transparent plate 262 is aunitary plate configured to cover an upper surface of the substratetable surrounding the substrate supporting area. Hence, a singleoptically transparent plate 262 may cover both the grating 50 (or sensor20) and the surface of the substrate table WT around the substrate W.The plate 262 may be held to the substrate table WT by a vacuum. Theplate 262 may be easily replaced. The plate 262 may be made of quartz.The heat transfer fluid channel 200 may be the same as described abovein relation to the channel 200 depicted in FIG. 15.

FIG. 17 is a schematic drawing, in cross-section, of a lithographyapparatus according to an embodiment of the invention. As depicted inFIG. 17, the grating 50 may be attached to the reference frame RF andthe sensor 20 may be attached to the substrate table WT. In this way,the grating 50 is suspended above the substrate table WT. Such a sensor20 and/or grating 50 may suffer from the same problem of a thermal loadbeing applied to its surface due to a droplet landing on its surface.The sensor 20 may be positioned at the corners of the substrate tableWT.

As depicted in FIG. 17, the heat transfer fluid channel 200 may bepositioned below the grating 50. The channel 200 may be positioned abovethe sensor 20 of the substrate table WT.

FIG. 18 depicts an embodiment in which the grating 50 is positionedbetween the plate 262 and the heat transfer fluid channel 200. Thisembodiment is equally applicable to the sensor 20, in place of thegrating 50. This embodiment is applicable to the context in which thegrating 50 (or sensor 20) is positioned on the upper surface of thesubstrate table WT. In this embodiment, the heat transfer fluid canthermally condition the rear side of the grating 50. The purpose of thisis that the radiation that is diffracted and/or reflected by the grating50 and detected by the sensor 20 does not pass through the heat transferfluid. Such control of the temperature of the grating 50 by having theheat transfer fluid channel at the rear side (i.e. the side opposite theplate 262 through which the measurement radiation passes) is equallyapplicable to the case in which the grating 50 or sensor 20 is attachedto the reference frame RF.

FIG. 19 depicts an embodiment to compensate for irregularities inthickness of the optically transparent plate 262. The thickness of theplate 262 is desirably uniform in order to ensure optical stability andconsistency at different locations of the plate 262. It can be difficultto manufacture a plate 262 with the required uniformity of thickness.Irregularity of thickness can be compensated for by use of markers 191at a surface of the plate 262. As depicted in FIG. 19, markers 191 maybe positioned at the top and or bottom (desirably both) surfaces of theplate 262. In this way, it is possible to at least partly compensate forthe irregularities in the thickness of the plate 262. The markers 191may be etched into the surface of the place 262, for example.

The temperature of the surface of the grating 50 and/or sensor 20 may becontrolled by using one or more electrical heaters and/or Peltiercoolers on the reverse side of the grating 50 and/or sensor 20. One ormore transparent thin-film heaters may be attached to the plate 262and/or to the grating 50. Such a thin-film heater may be positioned oneither side of the plate 262 and/or grating 50.

FIG. 23 depicts an embodiment in which the grating 50 has the dimensionsof the combination of the grating 50 and cover plate 262 of FIG. 16. Inthis case the cover plate may not comprise a part of the substrate tableWT. The grating 50 comprises a plate with a grating pattern formed onone surface. The plate may perform the function of protecting thegrating pattern from the immersion liquid. The grating 50 may extendfrom an edge of the substrate W with only a gap therebetween. The heattransfer fluid channel 200 may be arranged as described above. Thechannel 200 may be embedded in the grating 50.

FIG. 20 is a schematic drawing, in cross-section, of a lithographicapparatus according to an embodiment of the invention. The lithographicapparatus comprises a substrate table WT to hold a substrate W and/or areference frame RF. A grating 50 is attached to one of the substratetable WT and the reference frame RF. The grating 50 diffracts and/orreflects radiation detected to measure a relative position between thesubstrate table WT and the reference frame RF. An optically transparentplate 262 is configured to cover at least a part of a surface of thegrating 50. A vacuum pump 201 is configured to maintain a gap 202between the grating 50 and the plate 262 at a pressure lower than anambient pressure so as to hold the plate 262 in position. The vacuumpump 201 holds the plate 262 to the substrate table WT. The plate 262does not come into contact with the grating 50. The vacuum pump 201 maybe replaced by another arrangement to create a vacuum, such as aventuri.

The purpose of this is to reduce thermal conduction between the plate262 and the grating 50. The gap 202 is evacuated between the grating 50and the plate 262. As well as reducing the thermal conduction in the gap202, the vacuum pump 201 is also used to clamp the plate 262. FIG. 20depicts a grating 50 on the substrate table WT with a low-pressureregion adjacent the grating 50. In an embodiment, a sensor 20 on thesubstrate table WT has a low-pressure region adjacent to it. The grating50 or sensor 20 may be attached to a reference frame RF.

The plate 262 may act like a pellicle, namely a protective cover toprotect against damage and dirt. This improves the robustness of themeasurement with respect to contamination at the surface of the plate262.

It is possible that under the force of the vacuum pump 201, the plate262 will deform in shape, i.e. bow. The deformation of the plate 262 canvary slowly over time. This may adversely affect measurements made bythe grating 50 and sensor 20. If the grating 50 is not level with thesurface of the substrate W, then the measurements may become susceptibleto Abbé acute errors. This problem may be overcome by the unitary plate262 depicted in FIG. 16. In this case, by having a plate 262 that coversboth the grating 50 and the inner part of the substrate table WT, thedeformation of the plate 262 due to the force of the vacuum pump 201 isreduced. This is because the plate 262 is supported as a cantilever onits inner side. Furthermore, the unitary plate 262 that surrounds thesubstrate supporting area reduces the possibility of immersion liquidleaking into the gap 202.

If the unitary plate 262 of FIG. 16 is not used, then a separate innercover 222 to cover the upper surface of the substrate table WTsurrounding the substrate supporting area may be provided. As a result,there is a gap between the inner cover 222 and the plate 262. This gapcan be sealed with a flexible material and/or an overlying sticker. Anextractor may be provided to extract liquid from the gap between theinner cover 222 and the plate 262.

FIG. 21 depicts, in cross-section, an embodiment of the invention. Thelithographic apparatus comprises a vacuum clamp 211 configured to clampthe grating 50 to the substrate table WT or the reference frame RF. Thevacuum pump 201 is configured to maintain the gap 202 between thegrating 50 and the plate 262 at a pressure higher than the pressurebetween the grating 50 and the substrate table WT or reference frame RF.The purpose of this is to reliably hold the grating 50 to the substratetable WT (or reference frame RF). The gap 202 may be maintained at apartial vacuum.

FIG. 22 depicts, in cross-section, a variation of an embodiment of theinvention. As depicted in FIG. 22, there is a purge opening 221configured to supply a flow of gas into the gap 202 between the grating50 and the plate 262. The purpose of this is to remove particles trappedin the gap 202. Such particles may be trapped in the gap 202particularly during assembly of the apparatus. Clean gas can flow overthe surface of the grating 50. This gas may be removed via a tube usedby the vacuum pump 201. Hence, contaminants can be removed with a flowof clean gas over the surface of the grating 50.

FIGS. 21 and 22 depict a grating 50 on the substrate table WT with alow-pressure region adjacent the grating 50. In an embodiment, a sensor20 on the substrate table WT has a low-pressure region adjacent to it.The grating 50 or sensor 20 may be attached to a reference frame RF.

An embodiment of the invention results in improved overlay performanceof the lithographic apparatus. This is because evaporative cooling ofthe immersion liquid will have a reduced cooling effect on the grating50 and/or sensor 20 and will therefore deform the grating 50 and/orsensor 20 to a lesser extent. An embodiment of the present inventionprovides easier maintenance of the apparatus. This is because when thelyophobic coating of the plate 262 degrades, then the whole plate 262can be replaced.

Although an embodiment of the present invention has been described abovewith reference to an immersion lithographic apparatus, this need notnecessarily be the case. Other types of lithographic apparatus maysuffer from uneven cooling (or heating) around the edge of a substrate.For example, in an EUV apparatus (extreme ultra-violet apparatus)heating due to the impingement of the projection beam can occur. Thiscan give a localized heating to the substrate rather in the same way asthe passage of the edge of a substrate under the localized liquid supplysystem can give a cooling effect. If the heat transfer fluid in thechannel 200 is given a small negative temperature offset with respect tothe desired temperature in a normal operating condition, all the heaterscan be on to obtain the desired temperature. A local cooling load canthen be applied by switching a heater off. In this circumstance it maybe that the localization of the heaters only at the edge of thesubstrate is too limited and that heaters may be additionally oralternatively be placed at different radial distances from the center ofthe substrate supporting area. However, the same principles as describedabove apply in this case also.

Therefore, as can be seen, an embodiment of the present invention can beimplemented in many types of immersion lithographic apparatus. Forexample, an embodiment of the invention may be implemented in an I-linelithographic apparatus.

In an embodiment, there is provided a substrate table to support asubstrate on a substrate supporting area, the substrate table comprisinga heat transfer fluid channel under at least a part of the substratesupporting area, and at least one heater and/or cooler at a locationunder the substrate supporting area to control thermally the heattransfer fluid in the channel.

In an embodiment, the substrate table comprises a plurality of heatersand/or coolers at locations under the substrate supporting area tocontrol thermally the heat transfer fluid in the channel. In anembodiment, each of the plurality of heaters and/or coolers areindividually controllable. In an embodiment, the substrate table furthercomprises a thermal control element extending around at least part of aperiphery of the substrate supporting area. In an embodiment, thethermal control element comprises a plurality of thermal controlelements extending around the periphery of the substrate supportingarea. In an embodiment, the substrate table further comprises aplurality of thermal control element temperature sensors, each of thethermal control elements being associated with at least one of theplurality of thermal control element temperature sensors. In anembodiment, the substrate table further comprises a plurality of channeltemperature sensors, each of the heaters and/or coolers being associatedwith at least one of the plurality of channel temperature sensors. In anembodiment, a plurality of the plurality of channel temperature sensorsare at locations under the substrate supporting area. In an embodiment,each associated channel temperature sensor is downstream of itsassociated heater and/or cooler. In an embodiment, at least one of theplurality of temperature sensors is embedded in material of thesubstrate table. In an embodiment, at least one of the channeltemperature sensors is connected to an electrical assembly to readmeasurements from the channel temperature sensor indirectly via anelectrically conductive coating on the component to which the channeltemperature sensor is applied. In an embodiment, the substrate tablefurther comprises a controller configured to control power provided tothe at least one heater and/or cooler based on feedback based on atemperature measured by a temperature sensor. In an embodiment, thesubstrate table further comprises a controller configured to controlpower provided to the at least one heater and/or cooler based onfeedforward control. In an embodiment, there is provided an immersionlithographic projection apparatus comprising the substrate tabledescribed herein. In an embodiment, the immersion lithographic apparatusfurther comprises a liquid handling system configured to provide aliquid to a localized area of the top surface of the substrate tableand/or substrate.

In an embodiment, there is provided a method of compensating for a localheat load in a lithographic projection apparatus comprising a substratetable configured to support a substrate on a substrate supporting area,the method comprising: controlling at least one heater and/or cooler ata location under the substrate supporting area to control thermally aheat transfer fluid in a heat transfer fluid channel under at least apart of the substrate supporting area.

In an embodiment, there is provided a lithographic apparatus comprising:a substrate table to hold a substrate; a reference frame; a gratingattached to one of the substrate table and reference frame; and a sensorattached to the other of the substrate table and reference frame, thesensor configured to detecting radiation diffracted and/or reflected bythe grating so as to measure a relative position between the substratetable and the reference frame, wherein the substrate table and/or thereference frame comprises a heat transfer fluid channel over or underthe grating and/or sensor to control thermally the grating and/orsensor.

In an embodiment, the lithographic apparatus further comprises anoptically transparent plate configured to cover at least a part of asurface of the grating and/or sensor. In an embodiment, the channel ispositioned between the plate and the grating and/or sensor. In anembodiment, the grating and/or sensor is positioned between the plateand the channel. In an embodiment, the substrate table is configured tosupport a substrate on a substrate supporting area; and the plate is aunitary plate configured to cover an upper surface of the substratetable surrounding the substrate supporting area. In an embodiment, thelithographic apparatus further comprises a vacuum clamp configured tohold the plate and the substrate table together. In an embodiment, theplate comprises a marker etched into its surface so as to compensate forirregularity in thickness of the plate.

In an embodiment, there is provided a lithographic apparatus comprising:a reference frame and/or a substrate table to hold a substrate; agrating attached to the substrate table or the reference frame, whereinthe grating is configured to diffract and/or reflect radiation detectedto measure a relative position between the substrate table and thereference frame; an optically transparent plate configured to cover atleast a part of a surface of the grating; and a vacuum pump configuredto maintain a gap between the grating and the plate at a pressure lowerthan an ambient pressure so as to hold the plate in position.

In an embodiment, the lithographic apparatus further comprises a vacuumclamp configured to clamp the grating to the substrate table or thereference frame, wherein the vacuum pump is configured to maintain thegap between the grating and the plate at a pressure higher than apressure between the grating and the substrate table or the referenceframe.

In an embodiment, the substrate table is configured to support asubstrate on a substrate supporting area; and the plate is a unitaryplate configured to cover an upper surface of the substrate tablesurrounding the substrate supporting area. In an embodiment, thelithographic apparatus further comprises a purge opening configured tosupply a flow of gas into the gap between the grating and the plate soas to remove particles trapped in the gap.

In an embodiment, there is provided a lithographic apparatus comprising:a substrate table to hold a substrate on a substrate supporting area;and a grating or sensor attached to the substrate table, wherein thegrating or sensor is a part of a positional system, wherein thesubstrate table comprises a heat transfer fluid channel under thesubstrate supporting area and adjacent the grating or sensor, so as tocontrol thermally the temperature of the substrate supporting area andthe grating or sensor.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm).

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, including refractiveand reflective optical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the embodiments of the invention maytake the form of a computer program containing one or more sequences ofmachine-readable instructions describing a method as disclosed above, ora data storage medium (e.g. semiconductor memory, magnetic or opticaldisk) having such a computer program stored therein. Further, themachine readable instruction may be embodied in two or more computerprograms. The two or more computer programs may be stored on one or moredifferent memories and/or data storage media.

The controllers described above may have any suitable configuration forreceiving, processing, and sending signals. For example, each controllermay include one or more processors for executing the computer programsthat include machine-readable instructions for the methods describedabove. The controllers may also include data storage medium for storingsuch computer programs, and/or hardware to receive such medium.

One or more embodiments of the invention may be applied to any immersionlithography apparatus, in particular, but not exclusively, those typesmentioned above, whether the immersion liquid is provided in the form ofa bath, only on a localized surface area of the substrate, or isunconfined on the substrate and/or substrate table. In an unconfinedarrangement, the immersion liquid may flow over the surface of thesubstrate and/or substrate table so that substantially the entireuncovered surface of the substrate table and/or substrate is wetted. Insuch an unconfined immersion system, the liquid supply system may notconfine the immersion liquid or it may provide a proportion of immersionliquid confinement, but not substantially complete confinement of theimmersion liquid.

A liquid supply system as contemplated herein should be broadlyconstrued. In certain embodiments, it may be a mechanism or combinationof structures that provides a liquid to a space between the projectionsystem and the substrate and/or substrate table. It may comprise acombination of one or more structures, one or more liquid inlets, one ormore gas inlets, one or more gas outlets, and/or one or more liquidoutlets that provide liquid to the space. In an embodiment, a surface ofthe space may be a portion of the substrate and/or substrate table, or asurface of the space may completely cover a surface of the substrateand/or substrate table, or the space may envelop the substrate and/orsubstrate table. The liquid supply system may optionally further includeone or more elements to control the position, quantity, quality, shape,flow rate or any other features of the liquid.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A substrate table to support a substrate on a substrate supportingarea, the substrate table comprising a heat transfer fluid channel underat least a part of the substrate supporting area, and at least oneheater and/or cooler at a location under the substrate supporting areato control thermally the heat transfer fluid in the channel.
 2. Thesubstrate table of claim 1, comprising a plurality of heaters and/orcoolers at locations under the substrate supporting area to controlthermally the heat transfer fluid in the channel.
 3. The substrate tableof claim 2, further comprising a plurality of channel temperaturesensors, each of the heaters and/or coolers being associated with atleast one of the plurality of channel temperature sensors, wherein aplurality of the plurality of channel temperature sensors are atlocations under the substrate supporting area.
 4. The substrate table ofclaim 3, wherein at least one of the plurality of temperature sensors isembedded in material of the substrate table.
 5. The substrate table ofclaim 2, further comprising a plurality of channel temperature sensors,each of the heaters and/or coolers being associated with at least one ofthe plurality of channel temperature sensors, wherein at least one ofthe channel temperature sensors is connected to an electrical assemblyto read measurements from the channel temperature sensor indirectly viaan electrically conductive coating on the component to which the channeltemperature sensor is applied.
 6. The substrate table of claim 1,further comprising a thermal control element extending around at leastpart of a periphery of the substrate supporting area.
 7. The substratetable of claim 6, wherein the thermal control element comprises aplurality of thermal control elements extending around the periphery ofthe substrate supporting area.
 8. A method of compensating for a localheat load in a lithographic projection apparatus comprising a substratetable configured to support a substrate on a substrate supporting area,the method comprising: controlling at least one heater and/or cooler ata location under the substrate supporting area to control thermally aheat transfer fluid in a heat transfer fluid channel under at least apart of the substrate supporting area.
 9. A lithographic apparatuscomprising: a substrate table to hold a substrate; a reference frame; agrating attached to one of the substrate table and reference frame; anda sensor attached to the other of the substrate table and referenceframe, the sensor configured to detecting radiation diffracted and/orreflected by the grating so as to measure a relative position betweenthe substrate table and the reference frame, wherein the substrate tableand/or the reference frame comprises a heat transfer fluid channel overor under the grating and/or sensor to control thermally the gratingand/or sensor.
 10. The lithographic apparatus of claim 9, furthercomprising an optically transparent plate configured to cover at least apart of a surface of the grating and/or sensor.
 11. The lithographicapparatus of claim 10, wherein the channel is positioned between theplate and the grating and/or sensor.
 12. The lithographic apparatus ofclaim 10, wherein the grating and/or sensor is positioned between theplate and the channel.
 13. The lithographic apparatus of claim 10,wherein: the substrate table is configured to support a substrate on asubstrate supporting area; and the plate is a unitary plate configuredto cover an upper surface of the substrate table surrounding thesubstrate supporting area.
 14. The lithographic apparatus of claim 10,further comprising a vacuum clamp configured to hold the plate and thesubstrate table together.
 15. The lithographic apparatus of claim 10,wherein the plate comprises a marker etched into its surface so as tocompensate for irregularity in thickness of the plate.
 16. Alithographic apparatus comprising: a reference frame and/or a substratetable to hold a substrate; a grating attached to the substrate table orthe reference frame, wherein the grating is configured to diffractand/or reflect radiation detected to measure a relative position betweenthe substrate table and the reference frame; an optically transparentplate configured to cover at least a part of a surface of the grating;and a vacuum pump configured to maintain a gap between the grating andthe plate at a pressure lower than an ambient pressure so as to hold theplate in position.
 17. The lithographic apparatus of claim 16, furthercomprising a vacuum clamp configured to clamp the grating to thesubstrate table or the reference frame, wherein the vacuum pump isconfigured to maintain the gap between the grating and the plate at apressure higher than a pressure between the grating and the substratetable or the reference frame.
 18. The lithographic apparatus of claim16, wherein: the substrate table is configured to support a substrate ona substrate supporting area; and the plate is a unitary plate configuredto cover an upper surface of the substrate table surrounding thesubstrate supporting area.
 19. The lithographic apparatus of claim 16,further comprising a purge opening configured to supply a flow of gasinto the gap between the grating and the plate so as to remove particlestrapped in the gap.
 20. A lithographic apparatus comprising: a substratetable to hold a substrate on a substrate supporting area; and a gratingor sensor attached to the substrate table, wherein the grating or sensoris a part of a positional system, wherein the substrate table comprisesa heat transfer fluid channel under the substrate supporting area andadjacent the grating or sensor, so as to control thermally thetemperature of the substrate supporting area and the grating or sensor.